I was removing the background of a photo (painting it white actually) and now that I look at it I have painted a little bit too much over a persons head and I want that colour back. I know I could click 'undo' 100 times but would lose a lot of other work in the process - is there an easier way? I have Elements 9 :-)

If you still have the original you can select on it the portion of the head that you painted over. You don't have to be precise about the selection; just use the Lasso tool and roughly select around the portion. Then Edit > Copy.

Open the version you are working on, and Edit > Paste. The places the portion on a new layer. Use the Move tool to position this over the painted area; use arrow keys to fine-tune the position. Then use the Eraser with soft brush to remove excess from the selection.

In the method I described, when you copy/paste a portion of the orignal and attempt to move it to exactly the same place on the destination picture, it is not always easy to move it precisely. This especially is noticeble on faces. Here is an example where I deliberately positioned the pasted layer out of alignment.

Now here's a neat trick I learned from a PSE forum post long ago by Bob Warren:

1. Change the blending mode of the pasted layer to "Difference".

2. Use the arrow keys to nudge the pasted layer so that the result is all black.

3. Change the blend mode back to Normal and the 2 layers are now perfectly aligned!